Synthesis of oxygen-18 and deuterium labelled compounds are being developed for clinical studies on their metabolic fate and for internal standards in mass spectral assays. General synthetic reactions are being studied in an effort to provide key labelled intermediates in high yield. Synthesis of dioxygen-18 labelled 3,4 dihydroxphenylethyleneglycol (DHPG) has been completed on a test scale.